Bovine and frog rod outer segment preparations catalyze the transfer of N-acetylgalactosamine to rhodopsin. This sugar is not found in rhodopsin as it is ordinarily isolated and purified but may represent a transient component found only in the plasma membrane rhodopsin. As such, it could serve as a specific marker to mediate cell surface interactions such as the phagocytosis of shed outer segment tips by the pigment epithelium.